Process for the preparation of urea

ABSTRACT

The invention relates to a process for the preparation of urea from ammonia and carbon dioxide, the preparation being effected in whole or in part in a vertical combi-reactor. The gas stream leaving the stripper is fed to the condenser section of a vertical combi-reactor in which this gas stream is wholly or partially condensed in the carbamate stream which is transferred from the scrubber section to the condenser section via a downcomer. Ammonia and carbon dioxide are partially converted into urea in this condenser section of the combi-reactor. The urea conversion is completed in the reaction section of the combi-reactor.

[0001] The invention relates to a process for the preparation of ureafrom ammonia and carbon dioxide.

[0002] Urea can be prepared by introducing ammonia and carbon dioxideinto a synthesis zone at a pressure of between 12 and 40 MPa and at atemperature of between 150 and 250° C. Urea formation might best berepresented here by two consecutive reaction steps with ammoniumcarbamate being formed in the first step according to the exothermicreaction:

nNH₃ +CO ₂→H₂N—CO—ONH₄+(n−2)NH₃

[0003] Dehydration in the second step of the ammonium carbamate formedthen results in the formation of urea according to the endothermicequilibrium reaction:

H₂N—CO—ONH₄→H₂N—CO—NH₂+H₂O

[0004] The extent to which these reactions proceed depends on, amongother factors, the temperature and the excess ammonia used. A solutionthat consists essentially of urea, water, unbound ammonia and ammoniumcarbamate is obtained as reaction product. The ammonium carbamate andthe ammonia are removed from the solution and are preferably returned tothe synthesis zone. In addition to the aforementioned solution, a gasmixture is formed in the synthesis zone, which consists of non-convertedammonia and carbon dioxide plus inert gases. Ammonia and carbon dioxideare removed from this gas mixture and are preferably also returned tothe synthesis zone. The synthesis zone may comprise separate zones forthe formation of ammonium carbamate and urea. These zones may, however,also be united in a single apparatus.

[0005] In practice, various processes are used for the preparation ofurea. At first, urea was prepared in so-called conventionalhigh-pressure urea plants, which at the end of the 1960s were succeededby processes carried out in so-called urea stripping plants.

[0006] The conventional high-pressure urea plants that are currentlystill operating are understood to be urea plants in which thedecomposition of the ammonium carbamate not converted into urea and theexpulsion of the usual excess ammonia take place at a substantiallylower pressure than the pressure in the synthesis reactor itself. In aconventional high-pressure urea plant the synthesis reactor is usuallyoperated at a temperature of 180-250° C. and a pressure of 15-40 MPa.Furthermore, in a conventional high-pressure urea plant ammonia andcarbon dioxide are fed directly to the urea reactor. In a conventionalhigh-pressure urea process the molar NH₃/CO₂ ratio (=N/C ratio) in theurea synthesis lies between 3 and 6.

[0007] As regards the recycling of unconverted ammonia and carbondioxide to the synthesis section, one can distinguish Once Throughconventional urea plants(no recycle), Partial Recycle conventional ureaplants (only a proportion of ammonia and/or carbon dioxide is recycled)and Total Recycle plants (both ammonia and carbon dioxide are recycled).

[0008] A urea stripping plant is understood to be a urea plant in whichthe decomposition of the ammonium carbamate that has not been convertedinto urea and the expulsion of the usual excess ammonia largely takeplace at a pressure that is essentially almost equal to the pressure inthe synthesis reactor. This decomposition and expulsion take place inone or more stripper(s) installed downstream of the synthesis reactorwith the aid of a stripping gas, such as, for example, carbon dioxideand/or ammonia, and with addition of heat. It is also possible to applythermal stripping. Thermal stripping means that ammonium carbamate isdecomposed and the ammonia and carbon dioxide present are removed fromthe urea solution exclusively by means of the supply of heat. Theammonia and carbon dioxide-containing gas stream exiting from thestripper is condensed in a high-pressure carbamate condenser.

[0009] The gas mixture that has not reacted in the urea synthesis isvented from the synthesis section. In addition to the condensableammonia and carbon dioxide, this gas mixture (reactor vent gas) alsocontains inert gases. The condensable components (ammonia and carbondioxide) can be absorbed, for example, in a high-pressure scrubber atsynthesis pressure, before the inert gases are vented. In such ahigh-pressure scrubber the condensable components, ammonia and carbondioxide, are preferably absorbed from the reactor vent gas into thelow-pressure carbamate stream formed in the further recovery. Thecarbamate stream from the high-pressure scrubber, which contains theammonia and carbon dioxide absorbed from the reactor vent gas, isreturned to the synthesis whether or not via the high-pressure carbamatecondenser. The reactor, high-pressure scrubber, stripper andhigh-pressure carbamate condenser are the most important elements of thehigh-pressure section of a urea stripping plant.

[0010] In a urea stripping plant the synthesis reactor is operated at atemperature of 160-240° C. and preferably at a temperature of 170-220°C. The pressure in the synthesis reactor is 12-21 MPa, preferably12.5-19 MPa. The N/C ratio in the synthesis of a stripping plant liesbetween 2.5 and 5. The synthesis can be carried out in a single reactoror in a plurality of reactors arranged in parallel or in series. Whenuse is made of two reactors in parallel, the first reactor can beoperated using virtually fresh raw materials and the second using rawmaterials entirely or partly recycled, for example from the urearecovery.

[0011] A frequently used embodiment for the preparation of ureaaccording to a stripping process is the Stamicarbon CO₂-strippingprocess as described in European Chemical News, Urea Supplement, of Jan.17, 1969, pages 17-20.

[0012] The high-pressure carbamate condenser in a Stamicarbon CO₂stripping process is preferably designed as a so-called submergedcondenser as described in NL-A-8400839. The submerged condenser can beinstalled in horizontal or vertical position. It is, however,particularly advantageous to carry out the condensation in a horizontalsubmerged condenser (a so-called pool condenser; see for exampleNitrogen No 222, July-August 1996, pp. 29-31), because, in comparisonwith other designs of this condenser, the liquid generally has a longerresidence time in the pool condenser. This results in the formation ofextra urea in the pool condenser. The amount of urea formed in the poolcondenser is higher than 30% of the theoretically possible amount ofurea formed.

[0013] After the stripping treatment, the pressure of the stripped ureasynthesis solution is reduced in the urea recovery and the solution isevaporated, after which urea is recovered. This produces a low-pressurecarbamate stream in the recovery. This low-pressure carbamate stream ispreferably returned via the high-pressure scrubber to the sectionoperating at synthesis pressure.

[0014] In the high-pressure carbamate condenser the gas stream from thestripper condenses in the carbamate stream coming from the high-pressurescrubber. The carbamate solution coming from the high-pressure carbamatecondenser is preferably passed to the synthesis reactor together withthe ammonia needed for the reaction.

[0015] In a particular embodiment of a urea stripping process thefunctions of the reactor, pool condenser and high-pressure scrubber arecombined in a single high-pressure vessel with the functionalities ofthese process steps being separated in this high-pressure vessel bylow-pressure internals designed for small pressure differences. Anexample of such an embodiment is described in Nitrogen No. 222,July-August 1996, pages 29-31, which describes the poolreactor, as doesU.S. Pat. No. 5,767,313. This poolreactor preferably is placed inhorizontal position.

[0016] The aim of the present invention is to provide an improvedprocess for the preparation of urea, which also entails lower investmentcosts.

[0017] The applicant has found an improved process for the preparationof urea from ammonia and carbon dioxide, which is characterised in thatthe preparation takes place wholly or partly in a verticalcombi-reactor.

[0018] This combi-reactor is comprised of a condenser section, reactorsection and scrubber section, with the condenser section most usuallybeing located beneath the reactor section and the scrubber section beingplaced above the reactor section. The conditions of temperature andpressure in the reactor, scrubber and condenser are virtually equal andare such that the combi-reactor is operated at high pressure. Thepressure preferably is between 12 and 22 MPa, in particular between 13and 21 MPa. The temperature is between 150 and 250° C., preferablybetween 170 and 200° C. The process for the preparation of urea fromammonia and carbon dioxide is characterised herein that the gas streamcoming from the stripper is fed to the condenser section of a verticalcombi-reactor. In particular, this gas stream is wholly or partlycondensed in the carbamate stream which passes from the scrubber intothe condenser section through a downcomer. The condenser preferably isof the submerged type. The gas stream coming from the stripper consistsessentially of ammonia and carbon dioxide. The stripping gases aredistributed, with the aid of for example a gas divider, in the bottom ofthe condenser and are wholly or partially condensed in the carbamateoriginating from the scrubber via a downcomer.

[0019] The gas/liquid mixture which evolves subsequently passes throughthe condenser tubes, where the exothermic carbamate reaction takesplace. Low-pressure steam is formed around the tubes as a result of theheat released in this exothermic carbamate reaction. A proportion of thecarbamate formed can be returned to the bottom of the condenser sectionwith the aid of a funnel.

[0020] Carbamate circulates in the condenser section as a result of thedensity difference between the carbamate stream in the downcomer and thecarbamate/gas mixture in the tubes. This ensures intimate mixing of thecarbamate in the condenser section and generates turbulence, which isfavourable for heat transfer. Residence time for the liquid carbamate inthe condenser is provided by designing this condenser as a submergedcondenser so that urea formation partly takes place already here.

[0021] The remaining carbamate coming from the condenser, along with theinerts and unconverted ammonia and carbon dioxide, are passed to thereactor section of the combi-reactor together with the urea that hasalready formed and water. The remaining part of the endothermic ureareaction takes place in this reactor section. The heat required issupplied by the exothermic carbamate reaction between unconvertedammonia and carbon dioxide from the condenser.

[0022] The reactor section is preferably designed as a high-pressurebubble column. This reactor section is preferably provided with meanswhich ensure that the liquid flows through the reactor substantially inplug flow. To that end, this reactor section is divided, preferably bymeans of sieve trays, into compartments of virtually equal capacity, sothat the reactor is a cascade-type reactor and hence plug flow isapproached. The sieve trays used may be of any one of the typesdescribed in the literature on urea production. The urea solution ispassed to a downstream reactor or to a downstream gas/liquid separator.

[0023] The inerts, which still contain free ammonia and carbon dioxide,are washed in the scrubber section with the low-pressure carbamatestream evolving in the further recovery. Unconverted ammonia and carbondioxide may be washed out either in whole or part in this scrubber. Theinerts may be cleared of any remaining ammonia and carbon dioxideoutside the combi-reactor.

[0024] The combi-reactor may be used in grassroots plants and inexisting plants.

[0025] The invention also relates to a method of improving andoptimising (revamping) existing urea plants by addition of acombi-reactor. Such addition may take place in conventional plants andstripping plants.

[0026] The combi-reactor is a vertical reactor and, as a result of suchvertical positioning, the reactor requires only limited floor area. Theavailable floor area often is limited especially in revamping projects,for which reason the combi-reactor is particularly suitable here. Thecombi-reactor also is an attractive alternative to, for example, thepoolreactor.

[0027] The combi-reactor may be employed in conventional urea plants forrevamping projects in Once Through, Partial Recycle or Total Recycleplants. In the case of revamps of conventional urea plants it ispreferred to add, besides the combi-reactor, a stripper and a gas/liquidseparator.

[0028] The advantage of the combi-reactor in revamping conventionalplants is the fact that capacity increases of 1500 to 4000 tonnes a daycan be achieved with steam consumption being comparable to the steamconsumption in stripping plants, i.e. about 925 kg of steam per tonne ofurea. This is a remarkable improvement for a conventional urea plant.The combi-reactor affords the possibility of adding both condensercapacity and reaction volume to the existing synthesis.

[0029] In revamping projects the combi-reactor is operated at a pressurewhich is somewhat lower than the pressure in the downstream existingreactor. The driving force needed for the reaction solution to flow fromthe combi-reactor to the downstream reactor is supplied by ahigh-pressure ammonia ejector. In this way, the combi-reactor may beplaced at ground level.

[0030] The urea reaction is completed in the downstream existingreactor. To the downstream urea reactor is added, if necessary forprocess reasons, a small proportion of the fresh carbon dioxide which isneeded for the exothermic carbamate reaction to proceed in this reactor,which reaction supplies the heat needed for the endothermic reaction. Inthe case of revamping projects in conventional urea plants, the ureasolution is passed to a high-pressure stripper and preferably ahigh-pressure CO₂ stripper to be newly installed, in which theunconverted carbamate dissociates into ammonia and carbon dioxide. Freshstripping gases and preferably fresh carbon dioxide and heat areemployed for such dissociation. The evolving stripping gases aredischarged to the combi-reactor along with the vapour evolving in thedownstream existing urea reactor.

[0031] The revamping principle for stripping plants is essentially thesame as that described above for revamping conventional urea plants.Here, too, the existing reactor functions as downstream urea reactor andthe high-pressure ammonia ejector is the driving force to overcome thepressure difference between the combi-reactor and the existing reactor.In this way, the combi-reactor may be placed at ground level instripping units too. However, a stripper and gas/liquid separator neednot be added in stripping plants. Here again the combi-reactor affordsthe possibility of adding condenser capacity and reaction volume to theexisting synthesis, increasing the capacity of stripping plants to 4000tonnes per day or more, without increased steam consumption per tonne ofurea produced. Essentially, the existing high-pressure carbamatecondenser, which may be a falling-film type condenser or a submergedcondenser, may be incorporated in this design. This revamping techniquefor stripping plants may also be employed for CO₂ stripping plants andNH₃ stripping plants as well as for stripping plants in which thermalstripping is applied.

[0032] The principle of grassroots plants with a combi-reactor isessentially the same as that described for the revamping of conventionalurea plants. However, a downstream reactor is not needed in grassrootsplants inasmuch as the urea reaction can go to completion already in thereaction section of the combi-reactor.

[0033] The invention also relates to a urea plant in which thehigh-pressure section substantially consists of a combi-reactor, agas/liquid separator and a stripper, preferably a CO₂ stripper.

[0034] Since in grassroots plants the combi-reactor is also placed atground level, the urea discharged is passed to gas/liquid separator viaa high-pressure ammonia ejector. The liquid discharge of this separatoris sent to a high-pressure CO₂ stripper in which the unconvertedcarbamate dissociates. The stripping gases which also contain the freshcarbon dioxide along with the off-gases from the downstream separatorare discharged to the condenser section of the combi-reactor.

[0035] A major advantage of the process of the invention is that it canbe embodied in a plant with substantially lower investment costs becausethe integration of a heat exchanger/condenser and scrubber in a reactorrequires fewer items of equipment and piping, which must be resistant tohigh pressures in a highly corrosive environment. A further advantage isthe installation at ground level, resulting in a lower plant structure.This offers further advantages in terms of investment and affordsimproved safety.

[0036] The conversion of carbamate into urea and water can be effectedby providing for long enough a residence time for the reaction mixturein the combi-reactor. The residence time will in general be longer than10 min, preferably longer than 20 min. The residence time will ingeneral be shorter than 2 hours, preferably shorter than 1 hour. At ahigher temperature and pressure in the combi-reactor, a shorterresidence time will usually suffice to obtain high conversion.

[0037] The combi-reactor generally is designed as a wide pipe with adiameter of between 1 and 5 meters, preferably between 2 and 4 m. Thelength of the combi-reactor generally is between 5-60 meters, preferablybetween 10 and 40 meters.

[0038] The combi-reactor generally is provided with means that ensurethat the liquid flows through the reactor substantially in plug flow. Tothis end the reactor is provided with for example a structured packing(in one or more locations) or with baffles that divide the reactor intocompartments. The compartments form a succession of continuously stirredtank reactors (CSTRs), as it were. CSTRs and compartments willhenceforth be referred to on a number of occasions for a goodunderstanding of the invention. The use of these terms is not meant tolimit the invention thereto.

[0039] The number of compartments in the combi-reactor asseries-arranged CSTRs preferably is greater than 2, in particulargreater than 5. The number of compartments as CSTR will in general beless than 40, preferably less than 20.

[0040] The compartments in the combi-reactor are preferably formed byvirtually horizontal baffles. The surface area of such bafflespreferably is not less than 50%, preferably not less than 85%, of thehorizontal cross-sectional area of the vertical reactor. The surfacearea of the baffles preferably is practically 100% of the horizontalsection of the reactor placed in vertical position.

[0041] The heat released in the reactor can be carried off by waterpassing through the heat exchanger tubes, in which process the water isconverted into low-pressure steam of 3-10 bar, preferably 4-7 bar. Theheat may also be carried of by passing through a process stream thatneeds to be heated, for example a urea solution to be concentrated at2-8 bar or a urea solution to be expanded at 15-40 bar. The heatexchanger is installed in the condenser section of the reactor. Thissection occupies 10-70%, preferably 20-50%, of the total length of thereactor.

[0042] The condensation zone and the heat exchanger of the combi-reactorpreferably are designed as a so-called submerged condenser. Here, aportion of the gas mixture to be condensed passes through the tube sideof a tubular heat exchanger, with a dilute carbamate solution passingthrough the shell side, the released heat of solution and condensationbeing carried off by a medium, for example water, flowing around thetubes, the water being converted into low-pressure steam.

[0043] The invention is explained in further detail by way of examplewith reference to the following figures, where FIG. 1 represents thestate of the art and FIGS. 2 to 5 represent embodiments of the presentinvention.

[0044]FIG. 1: a schematic representation of a section of a ureastripping plant based on the Stamicarbon CO₂ stripping process.

[0045]FIG. 2: A schematic representation of a section of a ureastripping plant based on the Stamicarbon CO₂ stripping process to whicha combi-reactor is added.

[0046]FIG. 3: A schematic representation of a section of urea plantbased on the Conventional principle, incorporating a combi-reactor and aCO₂ stripper.

[0047]FIG. 4: A schematic representation of a section of a new ureastripping plant based on the combi-reactor principle.

[0048]FIG. 5: A schematic representation of a section of a combi-reactorof the present invention.

[0049] In FIG. 1, R represents a reactor in a Stamicarbon CO₂ strippingplant in which carbon dioxide and ammonia are converted into urea. Theurea synthesis solution (USS) leaving the reactor is sent to a CO₂stripper (S), in which the USS is converted into a gas stream (SG) and aliquid stream (SUSS) by stripping the USS with CO₂. The gas stream (SG)leaving the CO₂ stripper consists substantially of ammonia and carbondioxide and the SUSS is the stripped USS. The stream containing thestripped urea synthesis solution SUSS is passed to the urea recovery(UR) where urea (U) is recovered and water (W) is discharged. Alow-pressure ammonium carbamate stream (LPC) is obtained in the UR andis fed to a high-pressure scrubber (SCR). In this scrubber the LPC iscontacted with the gas stream (RG) coming from the reactor andconsisting substantially of ammonia and carbon dioxide as well as inerts(non-condensable components) present in the carbon dioxide feed and theammonia feed. Normally heat is also transferred in the SCR. In thisexample, the enriched carbamate stream (EC) leaving the SCR is passed tothe high-pressure carbamate condenser (C) in which the SG stream iscondensed with the aid of EC. This condensation may also be effectedwithout adding EC to C; in that case it is logical for EC to be added tothe reactor R direct. The resulting high-pressure carbamate stream (HPC)is returned to the reactor. In this example, the fresh ammonia isrecycled via the high-pressure carbamate condenser (C) but may of coursebe admitted elsewhere in the R→S→C→R loop or in the R→SCR→C→R loop.

[0050] In FIG. 2, R represents a reactor in a Stamicarbon CO₂ strippingplant in which carbon dioxide and ammonia are converted into urea. Theurea synthesis solution (USS) leaving the reactor is passed to a CO₂stripper (S) in which the USS is converted into a gas stream (SG) and aliquid stream (SUSS) by stripping the USS with CO₂. The gas stream (SG)leaving the CO₂ stripper consists substantially of ammonia and carbondioxide and the SUSS is the stripped USS. The stream containing strippedurea synthesis solution SUSS goes to the urea recovery (UR) where urea(U) is recovered and water (W) is discharged. A low-pressure ammoniumcarbamate stream (LPC) is obtained in the UR and is fed to ahigh-pressure scrubber (SCR). In this scrubber the LPC is contacted withthe gas stream (CRG) coming from the combi-reactor and substantiallyconsisting of ammonia and carbon dioxide as well as inerts(non-condensable components) present in the carbon dioxide feed and theammonia feed. Heat may optionally be transferred in this high-pressurescrubber (SCR). The enriched carbamate stream (EC) leaving the SCR ispassed to the scrubber section of the combi-reactor. The evolvinghigh-pressure carbamate stream is contacted, via a downcomer, with theammonia and gaseous carbon dioxide in the condenser section of thecombi-reactor. The carbamate and the urea formed in this condensersection of the combi-reactor is passed to the reaction section of thecombi-reactor. The urea solution, which also contains unconvertedcarbamate (HPC), is passed to the reactor (R) with the aid of an ejectorwhich is powered by the ammonia necessary. The gas stream (RG) leavingthe urea reactor (R), which consists essentially of ammonia and carbondioxide as well as inerts, goes to the condenser section of thecombi-reactor.

[0051] In FIG. 3, R represents a reactor in a conventional urea plant.The reaction product leaving the reactor goes as a mixed stream (MS) tothe gas/liquid separator (GLS), where the reaction product is separatedinto a gas stream (RG) and a liquid stream (USS). This liquid stream,consisting essentially of urea, ammonium carbamate and water, is passedto the stripper (S) and is stripped with carbon dioxide with addition ofheat. The stripper gas (SG) leaving the stripper is combined with gasstream (RG), both of which consist essentially of ammonia and carbondioxide, and transferred to the condenser section of the combi-reactor.The stripped urea solution (SUSS) leaving the stripper is transferred tothe urea recovery, where urea (U) is recovered and water (W) isrecovered. In the urea recovery evolves also an aqueous low-pressurecarbamate stream (LPC), which is passed to the scrubber section of thecombi-reactor. The evolving high-pressure carbamate stream is contacted,via a downcomer, with the ammonia and gaseous carbon dioxide in thecondenser section of the combi-reactor. The carbamate and the ureaformed in this condenser section of the combi-reactor are passed to thereaction section of the combi-reactor. The urea solution, alsocontaining unconverted carbamate (HPC), is passed to the reactor (R)with the aid of an ejector which is powered by the ammonia necessary.

[0052] In FIG. 4, CR is a schematic representation of a combi-reactorwith condenser, reaction section and scrubber in a section of a newplant design. The reaction product evolving in the combi-reactor istransferred as a mixed stream (MS), with the aid of an ammonia-poweredejector, to a gas/liquid separator (GLS). Here, the reaction product isseparated into a gas stream (RG) and a liquid stream (USS). The gasstream (RG) consists essentially of ammonia and carbon dioxide and theliquid stream (USS) consists essentially of water, ammonium carbamateand urea. The liquid stream (USS) is stripped in the stripper (S) withcarbon dioxide with addition of heat. In this process evolves a gasstream (SG) consisting essentially of ammonia and carbon dioxide and aliquid stream (SUSS) consisting essentially of urea and water. The gasstream is passed to the condenser section of the combi-reactor togetherwith the gas stream (RG). The urea solution (SUSS) is converted in theurea recovery into urea (U), water (W) and low-pressure ammoniumcarbamate (LPC). This low-pressure carbamate (LPC) goes to the scrubbersection of the combi-reactor.

[0053]FIG. 5 shows a schematic representation of an embodiment of thecombi-reactor (CR). Here, 1 represents the wall of the combi-reactor, 2the scrubber section, 3 the reactor section and 4 the condenser section.A and B are steam vessels. The gases (SG) leaving the stripper (S) enterthe condenser at 5 and are distributed in the bottom of the condenser bygas divider 6. The reaction mixture leaves the combi-reactor as a mixedstream (MS) through pipe 7. The low-pressure carbamate (LPC) from theurea recovery (UR) enters the combi-reactor at 8. Carbamate from thescrubber section enters the condenser section via downcomer 9. A tube ofthe submerged condenser section is represented by 10. 11 indicates asuitable location for a funnel, although a funnel is not necessary. Thisfunnel ensures further circulation of liquid across the condensersection via the downcomer. This is advantageous in that it promotes heattransfer in the condenser section.

[0054] The invention is further illustrated by the following examples.

Comparative Example A

[0055] Table 1 below presents the compositions in percent by weight ofthe various streams for a Stamicarbon CO₂ stripping plant as shown inFIG. 1. The compositions indicate that almost all urea is converted inthe reactor (R) and that carbamate condenses in the high-pressurecondenser (C). TABLE 1 Stream Urea NH₃ CO₂ H₂O Inert USS 33.0 30.5 18.018.5 — CO₂ — — 96.0 0.5 3.5 SUSS 55.0  7.8 10.5 26.7 — SG — 41.0 54.53.5 1.0 NH₃ — 99.6 — 0.4 — HPC — 49.5 42.0 8.0 0.5 RG — 50.0 39.5 3.57.0 EC — 39.0 39.0 22.0 — LPC — 30.0 37.0 33.0 — Inert —  5.5  5.0 0.589.0 

Example 1

[0056] Table 2 below presents the compositions in percent by weight ofthe various streams for a Stamicarbon CO₂ stripping plant in which acombi-reactor has been added to wholly or partially replace thehigh-pressure carbamate condenser (C) as shown in FIG. 2. Thecompositions of the streams indicate that a substantial part of the ureareaction takes place in the combi-reactor and that a proportion or allof the carbamate condenses in the combi-reactor, too. The urea reactionwhich takes place in the combi-reactor (CR) may be deployed as acapacity increase of the urea plant or as a reduction of the steamconsumption of this urea plant if capacity is kept at the same level. Inthis example, this saves 100 kg of high-pressure steam (300° C. and 2.2MPa absolute) per tonne of product produced. TABLE 2 Stream Urea NH₃ CO₂H₂O Inert USS 35.5 29.5 16.5 18.5 — CO₂ — — 96.0 0.5 3.5 SUSS 55.0  7.810.5 26.7 — SG — 40.0 54.0 3.5 2.5 NH₃ — 99.6 — 0.4 — HPC 23.5 34.5 26.016.0 — CRG — 56.0 37.0 2.5 4.5 RG — 49.0 40.5 3.3 7.2 EC — 39.0 39.022.0 — LPC — 30.0 37.0 33.0 — Inert —  5.5  5.0 0.5 89.0 

1. Process for the preparation of urea from ammonia and carbon dioxide,characterised in that the preparation is effected wholly or partly in avertical combi-reactor.
 2. Process according to claim 1, characterisedin that in the combi-reactor the condenser section is located beneaththe reactor section and the scrubber section is placed above the reactorsection.
 3. Process according to either of claims 1 and 2, characterisedin that the pressure in the combi-reactor is between 12 and 22 MPa andthe temperature is between 150 and 250° C.
 4. Process according to anyone of claims 1-3, characterised in that the gas stream leaving thestripper is fed to the condenser section of a vertical combi-reactor. 5.Process according to any one of claims 1-4, characterised in that thegas stream which leaves the stripper is wholly or partially condensed inthe carbamate stream which is transferred from the scrubber section tothe condenser section via a downcomer.
 6. Process according to any oneof claims 1-5, characterised in that the reactor section of thecombi-reactor is provided with means which ensure that the liquid flowsthrough the reactor substantially in plug flow.
 7. Process according toany one of claims 1-6, characterised in that the residence time of thereaction mixture in the combi-reactor is longer than 20 minutes. 8.Process according to any one of claims 1-7, characterised in that theresidence time of the reaction mixture in the combi-reactor is shorterthan 1 hour.
 9. Process according to any one of claims 1-8,characterised in that the combi-reactor is designed as a pipe having adiameter of between 1 and 5 meters and a length of between 5 and 60 m.10. Process according to any one of claims 1-9, characterised in thatthe number of compartments in the combi-reactor as series-arranged CSTRsis greater than
 2. 11. Process according to any one of claims 1-10,characterised in that the number of compartments in the combi-reactor asseries-arranged CSTRs is less than
 20. 12. Process according to any oneof claims 1-11, characterised in that the condensation zone and heatexchanger of the combi-reactor are designed as a submerged condenser.13. Method of improving and optimising an existing urea plant,characterised in that a combi-reactor is added.
 14. Method according toclaim 13, characterised in that, in a conventional urea plant, astripper and gas/liquid separator are also added.
 15. Method accordingto claim 13, characterised in that a combi-reactor is added in a ureastripping plant.
 16. Urea plant, characterised in that the high-pressuresection consists essentially of a combi-reactor, a gas/liquid separatorand a stripper.
 17. Method, urea plant and process as substantiallydescribed in the specification, drawings and example.